Mercury-free-high-pressure gas discharge Lamp

ABSTRACT

A mercury-free high-pressure gas discharge lamp (HID [high intensity discharge] lamp) is described which is provided for use in automotive technology. To achieve improved lamp characteristics, in particular a substantially equal luminous efficacy in comparison with lamps of the same power and a mercury-free gas filling, as well as a highest possible burning voltage, the discharge vessel ( 1 ) is provided in its wall regions ( 10 ) which are lowermost in the operational position with a coating ( 15 ) which reflects at least a portion of the infrared radiation generated during operation, such that the temperature of the coldest spots, and in particular of the light-generating substances collected there, is raised, with the result that the light-generating substances can enter the gas phase in sufficient quantities also without mercury, and in particular with the use of a metal halide as a voltage-gradient generator.

The invention relates to a high-pressure gas discharge lamp (HID [highintensity discharge] lamp) which is in particular free from mercury andsuitable for use in automobile technology.

Conventional high-pressure gas discharge lamps contain on the one hand adischarge gas (usually a metal halide such as sodium iodide or scandiumiodide) which is the actual light-emitting material (light generator),and on the other hand mercury which primarily serves to form a voltagegradient and has the essential function of enhancing the efficacy andburning voltage of the lamp.

Lamps of this kind have come into widespread use because of their goodproperties and they are increasingly applied also in the field ofautomobile technology. It is also partly required in particular for thisapplication, however, that the lamps should contain no mercury forenvironmental reasons.

A general problem with mercury-free lamps is that a given lamp power incontinuous operation results in a lower burning voltage and accordinglyin a higher lamp current and a lower luminous efficacy.

U.S. Pat. No. 5,952,768 discloses a discharge lamp with a transparent,preferably dichroic coating which absorbs ultraviolet radiation andpreferably reflects infrared radiation so as to bring the coldestregions of the lamp to a higher temperature. The object of this is tomaintain a higher metal halide vapor pressure and to improve theefficacy, life, and color properties of the lamp.

Among the disadvantages of this is that this lamp still contains mercuryin its gas filling, so that it does not comply with the aboverequirements relating to its use in automotive technology.

It is an object of the invention to provide a high-pressure gasdischarge lamp which has a mercury-free gas filling and which is capableof achieving a luminous efficacy substantially corresponding to that oflamps which do contain mercury.

A further object is to provide a high-pressure gas discharge lamp whichhas a mercury-free gas filling and has a higher burning voltage than isgenerally achievable with mercury-free lamps.

In particular, the object is to provide a high-pressure gas dischargelamp with which at least one of the two objects mentioned above (higherefficacy and higher burning voltage) can be achieved without thenecessity of increasing lamp power or enlarging the external dimensionsof the outer bulb of the lamp.

It is also an object to provide a mercury-free high-pressure gasdischarge lamp which has a lumen maintenance usual for automotiveapplications, i.e. in which the luminous decrement over lamp life showsa similar gradient as in lamps which do contain mercury.

Finally, the object is in particular to provide a high-pressure gasdischarge lamp which is suitable for use in automotive technology.

According to claim 1, the object is achieved with a mercury-freehigh-pressure gas discharge lamp having a discharge vessel whichcomprises an at least substantially infrared-reflecting coating on itswall portions which are lowermost in the operational position, thedimensioning of said coating being chosen such that after switching-onof the lamp the temperature of the light-generating substances collectedon the coated wall portions is increased to the extent that saidsubstances enter the gaseous state at least substantially.

The temperature rise is substantially achieved in that the infraredradiation issuing from the light arc discharge is incident on the coatedwall portions and is reflected there, so that said radiation passestwice through the light-generating substances, which thus are heatedcorrespondingly more strongly. The heating may in addition be caused toa minor degree by any portions of the infrared radiation absorbed by thecoating, whereby the coated wall portions, and thus also thelight-generating substances deposited thereon, are additionally heated.

To optimize the lamp properties and to achieve as high a burning voltageand luminous efficacy as possible, the coating is accordinglydimensioned such that the light-generating substances enter the gaseousstate as much as possible, preferably fully.

It can also be achieved in this manner inter alia that either mercurycan be omitted without any replacement, or that an alternativevoltage-gradient generator, for example a suitable metal halide which isless environmentally unfriendly, is used instead of mercury, providedthat in any case the light-generating substances enter the gas phase ina sufficient quantity owing to the achieved higher temperature of thecoldest spots, whereby the luminous efficacy and the burning voltage ofthe lamp are further enhanced. This may additionally be supported by theintroduction of a rare gas (in particular xenon) with which the gaspressure in the discharge space is increased.

A further advantage of this solution is that it can also be applied todischarge lamps with mercury in their gas fillings, the efficacy ofwhich can be considerably increased thereby.

It should be noted here that a high-pressure gas discharge lamp is knownfrom U.S. Pat. No. 4,281,267 in which the discharge vessel is providedwith an approximately semi-circular reflecting coating, which maycomprise zirconium oxide, in the regions of the electrodes, i.e. theaxial ends. The object of this coating is to reduce multiple internalreflections. In the case of lamps with flat pinches, the coatingprovided on the latter in addition serves to reduce heat radiation. Theefficacy and the luminous flux of the lamp is to be increased thereby,i.e. a suitable vapor pressure is to be maintained in the lamp.

This publication, however, does not take into account or even mentionthe problems connected with an omission of mercury. Finally, theparticulars important for use in automotive technology relating to theincorporation situation, the coating of the lamp for cooperation withthe reflector (the light arc, in particular the hot spots, and the freeends of the electrodes must not be screened off from the reflector), andthe requirement for as constant an outer shape of the lamp as possibleare not taken into account, so that this publication is regarded asirrelevant.

The dependent claims relate to advantageous further embodiments of theinvention.

It is possible to influence the temperature balance in a desired mannerby means of the types of dimensioning mentioned in claim 2. Herein thelocation of the temperature rise is determined by the location or extentof the coating (there where the light-generating substances are at leastsubstantially deposited), while the degree of the temperature rise isadjusted by the packing density and by the size of the particles in thecoating material, as well as the thickness of the coating.

The embodiment of claim 3 has the particular advantage thatlight-reflecting properties can be achieved thereby—for example in thecase of a metal coating—, so that an improved focusing of the radiatedlight in the manner of a primary and a secondary reflector can beachieved in co-operation with an additional main reflector.

The embodiment of claim 4 has the particular advantage that themanufacturing process of the lamp itself need not be changed, but thatan additional manufacturing step is used for providing the coating onthe lamp which is otherwise manufactured in the usual manner. Inaddition, the reflected infrared radiation passes not only twice throughthe light-generating substance, but also twice through the coated wallregions, to which regions the coldest spot also belongs, as was notedabove, so that also the temperature thereof is raised.

The embodiment of claim 5 is capable of preventing light-generatingsubstances from migrating into the pinches upon switching-on of the lampand the accompanying heating-up, which would cause corrosion there ofthe molybdenum foils connected to the respective electrodes.

Claim 6 describes an embodiment with a preferred, particularly effectivecoating.

Claims 7 and 8 relate to voltage-gradient generators which are to bepreferably used instead of mercury and by means of which a particularlygood luminous efficacy of the lamp can be achieved, while claim 9describes an alternative possibility for achieving this object, inparticular a higher efficacy and burning voltage.

Further particulars, characteristics, and advantages of the inventionwill become apparent from the following description of preferredembodiments, which is given with reference to the drawing, in which:

FIG. 1 is a diagrammatic side elevation of a first embodiment;

FIG. 2 is a diagrammatic side elevation of a second embodiment;

FIG. 3 is a diagrammatic side elevation of a third embodiment; and

FIG. 4 is a diagrammatic elevation of the third embodiment viewed frombelow.

FIGS. 1 to 3 show high-pressure gas discharge lamps according to theinvention in the operational state. The lamps each comprise a dischargevessel 1 of quartz glass which encloses a discharge space 2 and whichmerges into quartz glass portions (pinches) 5 at its mutually opposedends.

The discharge space 2 is filled with a gas which is composed of adischarge gas emitting light radiation through excitation or dischargeas well as preferably a voltage-gradient generator, which may both bechosen from the group of the metal halides.

The light-generating substance is, for example, sodium iodide and/orscandium iodide, while the voltage-gradient generator used may be, forexample, zinc iodide and/or other substances instead of mercury.

Alternatively or additionally to the voltage-gradient generator, certainquantities of rare gases (for example xenon) may be introduced into thedischarge space 2 so as to increase the gas pressure and thus theefficacy and the burning voltage.

The free ends of electrodes 3, which are manufactured from a materialwith as high a melting temperature as possible such as, for example,tungsten, project into the discharge space 2 from the mutually opposedends thereof.

The respective other ends of the electrodes 3 are each connected to anelectrically conductive tape or foil 4, in particular a molybdenum foil,via which an electrical connection is achieved between the connectionterminals 6 of the discharge lamp and the electrodes 3. These ends ofthe electrodes 3 and the electrically conductive foil 4 are embedded inthe pinches 5.

The pinches 5 are preferably symmetrically arranged with respect to thedischarge vessel 1, i.e. they lie on the longitudinal axis thereof. Thishas the advantage that the external dimensions of the outer bulb of thelamp according to the invention need not be changed, which is ofparticular importance especially for the use of these lamps in motorvehicle headlights. In addition, the manufacture of a lamp withsymmetrical pinches is simpler and thus more cost-effective.

An arc discharge (luminous arc) is excited between the tips of theelectrodes 3 in the operational state of the lamp.

As was noted above, the gas filling of the high-pressure gas dischargelamp according to the invention preferably comprises one or severalsuitable metal halides as a voltage-gradient generator instead ofmercury. Said halides, however, have a comparatively low partial vaporpressure, which renders it necessary to change the temperature balancein the discharge vessel 1 so as to achieve substantially the sameluminous efficacy (luminous flux) as with the use of mercury as well asthe highest possible burning voltage. Upon switching-on of the lamp, infact, it is necessary in particular to raise the temperature of thelight-generating substances, which have accumulated in the solid stateon the wall regions 10 lowermost in the operational position of theswitched-off lamp, to such a degree that they enter the gas phase in asufficient quantity in the discharge space 2 after switching-on forachieving a luminous efficacy and burning voltage which are as high aspossible. A further difficulty here is that the lowermost wall regions10 are the coldest regions in the operational state of the lamp.

The change in the temperature balance should be achieved here without anincrease in lamp power, if at all possible.

These objects are substantially achieved by the coatings 15 (shownhatched) described below, which are preferably provided on the outersurfaces of the discharge vessel 2 and on portions of the pinches 5, oron the inner or outer surfaces of an outer bulb (not shown) whichsurrounds the discharge vessel.

It is preferred to provide the coating 15 on the discharge vessel 2because the edge of the coating can be attuned more exactly to thepositions of the electrode tips and of the arc discharge formed betweenthem there, which tips must not be screened off (in the desiredradiation direction) by the coating.

As is shown in FIGS. 1 to 4, the coating 15 extends substantially onlyover the wall regions 10 which are lowermost in the operational positionand over portions of the side wall of the discharge vessel 1, whereasthe upper wall regions 13 have no coating. The portions of the pinches 5adjoining the discharge vessel 2, by contrast, are provided with thecoating 5 over their entire circumference.

In detail, the coating 15 in the first embodiment of FIG. 1 extends overthe lower wall regions 10 and the lateral walls of the discharge vessel1, the coating edge extending below a connecting line between the twoelectrodes 3 and parallel to this line. The edge of the coating thenextends upwards in the direction of the transition between the dischargevessel 1 and the pinch 5 in the region of each electrode tip, said pinchbeing finally fully surrounded by the coating 15.

In the second embodiment shown in FIG. 2, the edge of the coating 15extends over the lateral walls of the discharge vessel 1 substantiallyin a V-shape from the uppermost transition point between the dischargevessel 1 and the pinch 5 in the direction of the lowermost point of thedischarge vessel 1.

In the third embodiment shown in FIGS. 3 and 4, the edge of the coating15 is directed more steeply downwards at the lateral walls of thedischarge vessel 1 away from said transition point, such that a portionof the lower wall region 10 is not covered by the coating. This isvisible in particular in FIG. 4, which is a plan view of the lower sideof the lamp.

Besides these three examples, edge gradients are obviously possible forthe coating which are modifications of the gradients shown, i.e. inwhich, for example, the distance of the edge of FIG. 1 to the connectingline between the electrodes is greater or smaller, or the steepness ofthe gradient of the edge of FIGS. 2 and 3 is greater or smaller, or inwhich the edges are not straight but curved.

It should additionally be heeded in the shaping of the coating, inparticular if the coating is to be substantially impermeable to visiblelight, that the light arc, in particular its hottest location (hotspot), as well as the electrode tips are not hidden or screened off withrespect to a reflector.

The coating is substantially formed by zirconium oxide (ZrO₂).Alternative materials may be used, however, for example Nb₂O₅ and Ta₂O₅,which have an even better infrared-reflecting power than ZrO₂, but whichare comparatively expensive. Another possibility, finally, would be theuse of SiO₂ in crystalline form.

The infrared radiation originating from the arc discharge is reflectedby the coating for a major portion and is absorbed for a smaller portionor not at all. The coated wall portions and the light-generatingsubstance deposited thereon are accordingly heated more strongly by thedouble passage of the infrared radiation than the portions free fromcoating during lamp operation. The reflectivity, and accordingly thedegree of heating, is essentially determined by the composition of thecoating 15, in particular its packing density and particle size, andalso substantially by its thickness.

The coating 15 is provided on those regions and with such a packingdensity, particle size, and thickness, that the light-generatingsubstance accumulated on the lowermost wall regions 10 and said wallregions themselves, which are also the coldest spots, are heated asstrongly as possible after switching-on of the lamp.

A luminous efficacy of the lamp can be achieved in particular with thecoating 15 thus dimensioned such as had been possible until nowsubstantially only with gas fillings containing mercury. Furthermore,the spectral characteristics and the color point of the generated lightand the lumen maintenance correspond substantially to those of lampswhich do contain mercury, which is of particular importance forautomotive applications.

The burning voltage of the lamp is also substantially increased by thecoating 15 in comparison with known mercury-free lamps, again independence on the layer thickness, particle size, and packing density.

A suitable coating of certain regions, possibly with different layerthicknesses, packing densities, and particle sizes, renders it possiblealso to achieve a particularly homogeneous temperature distribution overthe wall of the discharge vessel 1 and the pinches 5.

To clarify the improvements achievable with the lamp according to theinvention, various comparative examples will be given below ofmercury-free high-pressure gas discharge lamps which contain zinc iodideas a voltage-gradient generator. The measured values listed below wereobtained from lamps without outer bulbs. The incremental values listedin the fourth column remain substantially the same with the use of anouter bulb.

Table 1 shows the luminous efficacy for various lamp types withoutcoating in comparison with the luminous efficacy of said lamps with azirconium oxide coating provided in accordance with FIGS. 1 to 3, andthe respective differences between these luminous efficacies. TABLE 1without ZrO₂ with ZrO₂ Lamp type Δ [lm/W] Δ [lm/W] Δ [lm/W] B15T-1 54.461.0 6.6 B15T-2 55.4 59.8 4.4 B16T-1 78.8 85.5 6.7 B16T-2 74.3 80.2 5.9B16T-10 76.2 85.4 9.2 B18T-5 67.2 73.2 6.0 B18T-6 70.9 75.1 4.2 B18T-967.4 71.9 4.5 P1-4 83.2 88.9 5.7 A3P-7 63.2 68.9 5.7 A3P-9 62.9 69.5 6.6

Table 2 below juxtaposes the burning voltages of these lamp typeswithout and with the coating according to the invention mentioned above,as well as the differences between the two burning voltages resultingtherefrom. TABLE 2 without ZrO₂ with ZrO₂ Lamp type U [V] U [V] ΔU [V]B15T-1 49.0 51.0 2.0 B15T-2 45.0 53.2 8.2 B16T-1 33.0 34.7 1.7 B16T-232.8 35.6 2.8 B16T-10 31.9 34.5 2.6 B18T-5 44.3 47.1 2.8 B18T-6 42.147.7 5.6 B18T-9 43.4 47.2 3.8 P1-4 34.2 37.0 2.8 A3P-7 46.8 54.5 7.7A3P-9 48.6 55.2 6.6

Table 3, finally, lists the temperatures of the coldest spots for thesame lamp types without coating and with the coating according to theinvention mentioned above, with the resulting temperature differences.TABLE 3 without ZrO₂ with ZrO₂ Lamp type T_(min) [° C.] T_(min) [° C.]ΔT_(min) [° C.] B15T-1 863 869 6 B15T-2 856 868 12 B16T-1 856 866 10B16T-2 856 871 15 B16T-10 844 862 18 B18T-5 833 853 20 B18T-6 827 857 30B18T-9 831 858 27 P1-4 835 852 17 A3P-7 850 871 21 A3P-9 840 865 25

A luminous efficacy and/or burning voltage satisfactory for certainapplications may be achieved with the coating 15 according to theinvention also if mercury is omitted without replacement, if so desired,i.e. without the use of a voltage-gradient generator, or if certainquantities of rare gases (for example xenon) are introduced into thedischarge space 2 as an alternative to the voltage-gradient generator soas to raise the gas pressure.

It should finally be pointed out that the principle of the invention, bywhich the temperature of the coldest spot of the discharge vessel israised, is obviously also applicable to lamps which do contain mercuryand in which the environmental disadvantages of mercury are accepted. Inthis case, such a temperature rise may serve, for example, to increasethe luminous efficacy or to reduce the lamp power for a given efficacy.

1. A mercury-free high-pressure gas discharge lamp having a dischargevessel (1) which comprises an at least substantially infrared-reflectingcoating (15) on its wall portions (10) which are lowermost in theoperational position, the dimensioning of said coating (15) being chosensuch that after switching-on of the lamp the temperature of thelight-generating substances collected on the coated wall portions isincreased to the extent that said substances enter the gaseous state atleast substantially.
 2. A high-pressure gas discharge lamp as claimed inclaim 1, wherein the dimensioning of the coating is given by its surfacearea and/or thickness and/or particle size and/or packing density of theparticles.
 3. A high-pressure gas discharge lamp as claimed in claim 1,wherein the coating (15) is at least substantially impermeable tovisible light.
 4. A high-pressure gas discharge lamp as claimed in claim1, wherein the coating (15) is provided on the outer surfaces of thedischarge vessel (1) or on the inner or outer surfaces of an outer bulbsurrounding the discharge vessel.
 5. A high-pressure gas discharge lampas claimed in claim 1, wherein the coating (15) is provided on regionsof pinches (5), i.e. on those regions thereof which adjoin the dischargevessel (1).
 6. A high-pressure gas discharge lamp as claimed in claim 1,wherein the coating (15) is formed from zirconium oxide.
 7. Ahigh-pressure gas discharge lamp as claimed in claim 1, which comprisesin its gas filling a voltage-gradient generator in the form of one orseveral metal halides.
 8. A high-pressure gas discharge lamp as claimedin claim 7, wherein the voltage-gradient generator comprises zinciodide.
 9. A high-pressure gas discharge lamp as claimed in claim 1,wherein the gas filling comprises additional quantities of rare gasessuch as xenon so as to increase the gas pressure and to enhance theluminous efficacy of the lamp.
 10. A lighting unit, in particular formotor vehicle headlights, comprising a high-pressure gas discharge lampas claimed in claim 1.